chore(lib): Remove old validation check preventing `loop=True` with `auto_next=True`

[mateuszdrwal]

Description

299 added support for stopping looping slides smoothly, at which point specifying both loop=True and auto_next=True on a next_slide starts making sense. There is an old validation check in the code referencing #299 that was never removed.

This functionality (on the next press, waiting for the current loop to end and then immediately proceeding to the next slide) seems to work without issues on the QT presenter. The reveal.js presenter doesn't like it as much, but it's also not supported by --next-terminates-loop.

Check List (Check all the applicable boxes)

• [x] I understand that my contributions needs to pass the checks.
• [x] If I created new functions / methods, I documented them and add type hints.
• [x] If I modified already existing code, I updated the documentation accordingly.
• [x] The title of my pull request is a short description of the requested changes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.24%. Comparing base (9460f6b) to head (9f1d27d). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #445 +/- ## ========================================== - Coverage 79.30% 79.24% -0.07% ========================================== Files 22 22 Lines 1822 1816 -6 ========================================== - Hits 1445 1439 -6 Misses 377 377 ```

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[jeertmans]

Looks great, thanks!

[jeertmans]

Would you mind adding an entry to the CHANGELOG file?

[mateuszdrwal]

Yep, there you go

[jeertmans]

Yep, there you go

Many thanks!!